Determination of Inorganic Cations and Ammonium in Environmental Waters by Ion Chromatography Using the Dionex IonPac CS16 Column

Significance of the Topic

Monitoring alkali and alkaline earth cations along with ammonium in environmental waters is critical for assessing water hardness, corrosion potential and ecological toxicity. Conventional approaches require separate assays for ammonium and other cations. A unified ion chromatography method streamlines analysis, reduces sample preparation and enhances laboratory throughput.

Objectives and Overview

This study evaluates a single‐run ion chromatography method using a high‐capacity Dionex IonPac CS16 column with methanesulfonic acid eluent and suppressed conductivity detection to quantify lithium, sodium, ammonium, potassium, magnesium and calcium in drinking water, wastewater and soil extracts. Key goals include determining linear ranges, detection limits, precision, accuracy and matrix effects.

Methodology and Instrumentation

The system comprised a Dionex DX‐600 (or ICS‐2100/ICS‐5000+) with a 5×50 mm CG16 guard and 5×250 mm CS16 analytical column. Eluent was 26 mM methanesulfonic acid generated on‐line by an EG40 eluent generator or prepared manually. Detection used a CSRS ULTRA suppressor in recycle mode at 100 mA. Samples were filtered (0.45 µm) and injected (10 µL). Standards were prepared from 1000 mg/L stocks and combined for working solutions. Soil extracts were obtained by ultrasonic extraction of 3 g soil with water or eluent, followed by filtration. The suppressor was hydrated with NaOH, and columns equilibrated at 30 °C for stable retention times.

Main Results and Discussion

At 30 °C, all six cations were baseline resolved within 25 minutes. Retention time reproducibility was better than 0.2% RSD and peak area precision under 1% RSD at low mg/L levels. Calibration was linear over three orders of magnitude for most ions (0.05–80 mg/L) and up to 40 mg/L for ammonium using a quadratic fit. Method detection limits ranged from 1 to 10 µg/L. High column capacity (8400 µeq) enabled separation of trace ammonium in the presence of a 10,000-fold excess of sodium. Spike recovery experiments in reagent water, wastewater, drinking water and standard wastewater matrices showed 80–120% recovery for all ions. The CS16 phase tolerated acidified samples up to 100 mM hydronium ion without pH adjustment. Fouling agents were removed by flushing with acetonitrile when needed.

Benefits and Practical Application

• Unified determination of six cations and ammonium in a single run reduces labor and reagents.
• High capacity column handles samples with high ionic strength without compromising resolution.
• Automated eluent generation enhances reproducibility and lowers consumable costs.
• Suppressed conductivity detection achieves low µg/L detection limits.

Future Trends and Applications

• Broader use in regulatory monitoring of drinking water, wastewater discharge and soil leachates.
• Integration with mass spectrometry for speciation and enhanced sensitivity.
• Development of next‐generation high‐capacity stationary phases for faster separations.
• Miniaturized or field‐deployable IC systems for on‐site environmental screening.

Conclusion

The described IC method using the Dionex IonPac CS16 column, methanesulfonic acid eluent and suppressed conductivity detection provides accurate, precise and sensitive measurement of inorganic cations and ammonium across diverse environmental matrices. Its high capacity and automation-friendly design make it a valuable tool for routine water quality and compliance testing.

Reference

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6. EPA Method 300.0: Determination of Inorganic Anions by Ion Chromatography; U.S. EPA, Cincinnati, OH, 1993.